- The zerg on Char turning feral after Kerrigan's de-infestation.(src)
Feral zerg are zerg not under the control of any higher entity. These zerg will simply attack everything around them, acting on the lowest level, and behaving as predators, consuming other animals for nourishment. Control can be re-asserted over feral zerg by command strains however, even those as low as overlords.
During the Great War, the zerg in Zasz's Garm Brood went feral after Dark Prelate Zeratul slew Zasz on Char. These renegade zerg posed such a threat to the Swarm that Daggoth ordered the entire brood eradicated.
In the immediate aftermath of the Overmind's destruction, nearly all the zerg on Aiur became feral and went on a rampage killing everything in their path. Even after Sarah Kerrigan took control of the Swarm, the zerg on Aiur remained outside her direct control. When Zeratul returned to Aiur in 2504 to visit the Overmind's final resting place, he found many feral zerg guarding and feeding off its corpse. Likewise, the zerg on the planet Cavir remained feral after the death of their master Daggoth following the end of the Brood War.
In the aftermath of Kerrigan's de-infestation during the Second Great War, the Swarm splintered into multiple factions, and many zerg turned feral. The zerg on Char that had previously been guarding Kerrigan immediately turned feral and began devouring everything in sight, including the corpses of zerg killed by Raynor and his men during their escape.
A number of feral zerg creatures were kept at the Research Station EB-103 as test subjects. When the Terran Dominion attacked the station, many of these test subjects broke loose. During their escape from the station, Kerrigan and Raynor not only had to deal with the Dominion, but also the escaped ferals. This included a large pack of zerglings Kerrigan had recently created—ironically, to show Valerian Mengsk why it was a bad idea to conduct experiments with live zerg.
Feral zerg on Astrid III developed the swarmling-strain of the zergling. This adaptation caught the attention of Kerrigan, who seized control of the ferals and used them to destroy the local Dominion garrison.
Feral brood lords and mutalisks resided near the Dominion defense platform Sigma Centari. Kerrigan had her zerg kill these ferals in order to collect their essence and reacquire the brood lord strain for the Swarm.
For years, a flock of feral mutalisks lived in seclusion on the terran-occupied moon Choss, having survived the Dominion's earlier efforts to eradicate the zerg presence there. Among these ferals was a rare "canny mutalisk" that would later be nicknamed "Scar." During the Second Great War, the mutalisks began attacking the locals. Eventually, the colonists succeeded in exterminating the ferals.
After the End War, feral zerg began attacking Terran Dominion worlds, lured by the separatist group the Defenders of Man setting psi-emitters on Dominion worlds. They did to show the weakness of Emperor Valerian Mengsk's regime, and would swoop in to save the surviving population of the planet. They first did so with Antiga Prime, causing the world to be overrun. Nova Terra discovered this, but was mind wiped before she could notify the Dominion. When feral zerg attacked the planet of Borea, and Nova was forced to defend Admiral Matt Horner's base before it was overrun. However, the Defenders of Man arrived to clean out the zerg. To find out what the Defenders of Man were up to, Nova and her Covert Ops Crew moved to Tarsonis, which had been since overrun by feral zerg, to infiltrate a Defenders of Man base there. It was there she discovered the Defenders of Man were using psi emitters to lure feral zerg to Dominion planets.
The Defenders of Man lured a massive swarm of feral zerg to Tyrador IX, which caused massive Dominion military and civilian casualties. Nova and her Covert Ops Crew intervened and attacked the nearby feral zerg hives while the Defenders of Man began evacuating civilians. The feral zerg were then attacked by the Tal'darim Death Fleet under First Ascendant Ji'nara, who sought the destruction of the Defenders of Man for provoking the Tal'darim. Reigel noted strange mutations from the feral zerg in the region, which if gathered would help his research. After the Tal'darim were repelled, the Defenders of Man rallied and defeated the remaining feral zerg.
Nova later returned to Antiga Prime to recover her memories using terrazine via the bargain she struck with Highlord Alarak, but ran against the feral zerg fortifications. Reigel noted that if the zerg structures were destroyed it could stop the feral zerg from spawning specialist units. Nova was successful at defending from the zerg and escaping the planet with the information she needed.